What will happen when the universe stops expanding?
Most astronomers believe that it never will stop.
Most scientists believe that it will never stop, in fact its rate of expansion is thought to be increasing. But if there is enough mass in the Universe (composed mostly of dark matter) to eventually stop expansion the the universe would start to contract and eventually become a singularity as its was at the Big Bang.
If the universe stopped expanding it would eventually collapse into a singularity and another Big Bang. The empirical evidence is against this happening.
Before 1997 the consenus of scientific community was that the expansion of the universe would stop and that the material universe would recycle. It was postulated that there would be enough dark matter in the universe to create the gravitational force to slow , stop and reverse the expansion of the universe. This theory was consistent with the prevailing scientific philosophy of material realism or naturalism.
In 1997 a study of supernova was done to confirm the hypothesis that the expansion of the universe was slowing down. The results were the opposite of what was expected. Subsequently two other studies were done to confirm or deny the results of the first study.
All three studies provided strong empirical evidence that the rate of the expansion of the universe was increasing instead of decreasing as hypothesis predicted.
If the expansion of the universe were to slow and then stop the observable universe would recycle in a series of Big Bangs and Big Crushes. This would be consistent with the scientific philosophy that matter and energy are all that is , was or ever will be. However the empirical evidence now leads most of the scientific community convinced that the universe will never stop expanding.
The scientific community has yet to resolve the conflict between the empirical evidence and the prevailing scientific philosophies.